[metif12]

split: implement it

src/split held a delete.me and nothing else, so split was a binary that did
nothing. Of the 96 directories here, 33 are like that.

What it does now: writes a file into fixed sized pieces, named PREFIXaa, PREFIXab
and so on, or PREFIX00 with -d, and widens the suffix when the combinations at the
current width run out rather than failing.

Measured against GNU 9.4

Twenty one cases, each running this binary and GNU's over the same input and
comparing the names produced, every file's contents and the exit status:

default                      same
-l 3                         same
-d -l 3                      same
-a 3 -l 3                    same
-l 1                         same
-l 4 -d                      same
empty input                  same
no trailing newline          same
-b 4                         same
-b 5                         same
-b 10                        same
-b 1                         same
-C 4                         same
-C 5                         same
-C 1                         same
-b 4 -a 1                    same
explicit prefix              same
two ways asked for at once   same
-l 0, -b 0                   same
no such file                 same

The two modes that are easy to get wrong are pinned down rather than guessed:

• -b N cuts where the byte count says, mid line if that is where it lands. With
  -b 1 over a 21 byte file GNU writes 21 files of one byte, and so does this.
• -C N is that cut with one difference: a line is never split, so a line that does
  not fit starts the next file. With -C 4 over ten two byte lines GNU puts four
  lines in a file and gives the last two a file each, because "9\n10\n" is 5 bytes;
  with -C 5 they share one file. With -C 1, where no line fits at all, GNU stops
  trying and cuts by bytes instead, giving the same 21 one byte files as -b 1. That
  last one was wrong here at first: this wrote whole lines, and comparing showed it.

A missing input file is reported with the same wording as GNU:

split: cannot open 'nothere.txt' for reading: No such file or directory

What is left, and why it is left

-n is not implemented, and says so rather than producing something plausible. It
takes two forms and neither is the even split the word "chunks" suggests: over ten
lines, -n l/7 gives 2,1,2,1,2,1,1 and -n 7 gives 2,1,2,1,2,1,1 lines. Three data
points are not enough to say what the rule is for every N, and a split that puts the
wrong bytes in the wrong file is worse than one that refuses.

--filter is not implemented for a different reason. It runs a command per piece, and
on Windows the only way this V has of starting a process from a command string is
os.system, which is deprecated and an error under -W, which is what make test
builds with. Shipping it would mean shipping a deprecated call.

One known difference in argument handling: -n 0 says "chunks are not implemented"
where GNU says "invalid number of chunks: ‘0’", and GNU quotes the number with the
curly quotes that this port uses straight ones for. The number is not parsed before
the refusal, so an invalid -n is reported the same way as a valid one.

What did not change

Nothing else. delete.me stays where it is, as it does in the other 32 directories
that hold one.